Surface-tension-driven flows of coatings : Bondline readout formation

Bondline readout (BLRO) is a coating defect frequently observed on adhesively bonded, polymeric automotive body panels. This paper addresses ridging BLRO in clearcoats, not optical and mechanical BLRO, which are characterized by metal-flake orientation (dark /light effects) in basecoats and by distortion in substrates, respectively. Ridging BLRO is due to film thickness differences and results from Marangoni-type, surface-tension-driven flows. In this study, the effects of several parameters on BLRO are investigated experimentally. These parameters include initial film thickness, heating rate, viscosity, solvent-to-resin surface-tension ratio, and solvent volatility. The experiments clearly demonstrate three modes of BLRO flow - formation, flow-out, and reformation - that result from competing surface-tension-gradient forces (temperature - versus concentration-induced). Experimental results are used to validate a proposed BLRO mechanism and, in subsequent work, a BLRO-predicting numerical code.